1. Technical Field
The present invention relates to a surface acoustic wave resonator and a surface acoustic wave oscillator in which the surface acoustic wave resonator is mounted, and more particularly, to a type of surface acoustic wave resonator in which grooves are arranged in a substrate surface and a surface acoustic wave oscillator and an electronic apparatus in which such a type of surface acoustic wave resonator is mounted.
2. Related Art
In a surface acoustic wave (SAW) device (for example, a SAW resonator), the influence of the stop band of a SAW, a cut angle of a piezoelectric substrate (for example, a quartz crystal substrate), the formed shape of an IDT (inter-digital transducer), and the like on the change in the frequency-temperature characteristics is great.
For example, in Japanese Patent No. 3266846 (JP-A-11-214958), a configuration used for exciting an upper end mode and a lower end mode of a stop band of the SAW, distributions of standing waves in the upper end mode and the lower end mode of the stop band, and the like are disclosed.
In addition, in JP-A-2002-100959, JP-A-2006-148622, JP-A-2007-208871, JP-A-2007-267033, and JP-A-2007-300287, the frequency-temperature characteristics of the stop band upper end mode of the SAW that are better than those of the stop band lower end mode are described.
Among them, in JP-A-2002-100959, the frequency-temperature characteristics are described to be improved by using the resonance of the upper end of the stop band using a rotated Y-cut X-propagation crystal quartz substrate more than a case where the resonance of the lower end of the stop band is used.
In addition, in JP-A-2006-148622 and JP-A-2007-208871, in order to acquire good frequency-temperature characteristics from a SAW device using a Rayleigh wave, adjusting the cut angle of the quartz crystal substrate and thickening the normalized electrode film thickness (H/λ) up to 0.1 are described to be performed. Here, λ is the wavelength of the SAW.
Furthermore, in JP-A-2007-267033, adjusting the cut angle of the quartz crystal substrate and thickening the normalized electrode film thickness (H/λ) so as to be equal to or greater than 0.045 are described to be performed for a SAW device using a Rayleigh wave.
In JP-B-2-7207 (JP-A-57-5418) and “Manufacturing Conditions and Characteristics of Groove-Type SAW Resonator” (The Institute of Electronics, Information and Communication Engineers Technical Research Report MW82-59 (1982)), in a SAW device using an ST-cut quartz crystal substrate, grooves are described to be arranged between electrode fingers configuring the IDT and between conductive strips configuring a reflector. In addition, in “Manufacturing Conditions and Characteristics of Groove-Type SAW Resonator” described above, the frequency-temperature characteristics are described to change in accordance with the depth of the grooves.
In addition, also in JP-A-2-189011, JP-A-5-90865, JP-A-1-231412, and JP-A-61-92011, the frequency characteristics are described to be adjusted by forming grooves in a piezoelectric substrate formed from quartz crystal or the like.
Furthermore, it is disclosed in JP-A-10-270974 that, in a transversal-type SAW filer, grooves are formed by etching and processing the surface of a piezoelectric substrate between electrodes of an IDT, and by forming electrode fingers of pure metal or an alloy that has specific gravity higher than that of aluminum, the appearing propagation speed is reduced so as to decrease the pitch of the electrode fingers, whereby realizing the miniaturization of a corresponding chip.
In JP-B-1-34411, implementation of third-order frequency-temperature characteristics by exciting a SSBW (Surface Skimming Bulk Wave) in a SAW resonator that is formed by forming an IDT electrode, of which a normalized electrode film thickness (H/λ) is in the range of 2.0≦H/λ≦4.0%, from aluminum in a quartz crystal substrate having a rotated Y-cut, a cut angle of −43° or −52°, and the slip wave propagation direction set in the Z′-axis direction (Euler angles (φ, θ, ψ)=(0°, 38°≦θ≦47°, 90°)) is disclosed. However, this SAW resonator has features in that an SH wave propagating right below the surface of a piezoelectric substrate is excited by an IDT, and the vibrational energy is confined right below the electrode, and accordingly, the SH wave is basically a wave that progresses inside the substrate. Therefore, the reflection efficiency of the SAW using a grating reflector is lower than that of an ST-cut quartz crystal SAW propagating along the surface of the piezoelectric substrate, and there is a problem in that a miniaturized SAW device having a high Q value cannot be easily implemented.
In addition, in PCT Republication No. WO2005/099089 A1, in order to solve the above-described problems, a SAW device acquired by forming an IDT electrode and a grating reflector on the surface of a quartz crystal substrate having Euler angles (φ, θ, ψ)=(0°, −64°<θ<−49.3°, 85°≦ψ≦95°) is proposed.
Furthermore, in JP-A-2006-203408, in consideration of a problem in that a Q value or the frequency stability deteriorates due to stress migration that occurs due to a large electrode film thickness, it is disclosed that grooves are formed on a crystal quartz substrate located in an area corresponding to a space between electrode fingers through an etching process, and when the depth of the grooves is Hp, and the film thickness of the metal film is Hm, a normalized electrode film thickness (H/λ) is set in the range of 0.04<H/λ<0.12 (here, H=Hp+Hm). Accordingly, a SAW device, in which the variation in the frequency is suppressed, having a high Q value can be realized.
In JP-A-2009-225420, in a SAW device using a so-called in-plane rotation ST quartz crystal substrate, disclosed in JP-A-2006-148622, JP-A-2007-208871, JP-A-2007-267033, and JP-A-2007-300287, considering that side etching progresses in a process of forming electrode fingers through etching due to a large electrode film thickness, an individual line occupancy ratio varies, and the amount of change in the frequency is large when the temperature changes, so as to cause serious problems in the reliability and the quality of the product, it is proposed to use an in-plane rotation ST-cut quartz crystal having Euler angles (φ, θ, ψ)=(0°, 95°≦θ≦155°, 33°≦|ψ|≦46°). By using this quartz crystal substrate, by exciting the upper limit mode of the stop band of the surface acoustic wave, a SAW device suppressing the unbalance in the frequency variation can be implemented.
However, although the unbalance of the variation in the frequency can be suppressed while securing an effective film thickness by forming the grooves by etching the surface of the quartz crystal substrate between the electrode fingers as above, the frequency-temperature characteristics in the operating temperature range of the SAW device still have second-order characteristics, and the width of the variation in the frequency is not decreased much.
In addition, in Japanese Patent No. 3851336, while a configuration for forming a curve representing the frequency-temperature characteristics as a third-order curve in a SAW device using an LST-cut quartz crystal substrate is described, a substrate having a cut angle representing the temperature characteristics as represented by a third-order curve is described to have not been found in a SAW device using a Rayleigh wave.
As described above, factors for improving the frequency-temperature characteristics include many things, and, particularly in a SAW device using a Rayleigh wave, forming the film thickness of the electrode configuring the IDT to be large is considered as one factor contributing to the frequency-temperature characteristics. However, the applicants of this application has found through experiments that, when the film thickness of the electrode is formed to be large, environment-resistant characteristics such as characteristics that change by time or temperature-resistant shock characteristics deteriorate. In addition, in a case where the main purpose is to improve the frequency-temperature characteristics, the film thickness of the electrode needs to be large as described above, and it is difficult to avoid the deterioration of the characteristics changing by time and the temperature-resistant shock characteristics that is accompanied with the large film thickness. This can be also applied to the Q value, and it is difficult to realize a high Q value without forming the film thickness of the electrode to be large. In addition, by forming the film thickness of the electrode to be large, a CI (crystal impedance) value also increases, whereby the stability of oscillation is degraded.
Therefore, aspects of the invention for providing a surface acoustic wave resonator, a surface acoustic wave oscillator, and an electronic apparatus are, first, to realize good frequency-temperature characteristics, second, to improve the environment-resistant characteristics, third, to acquire a high Q value, and, fourth, to acquire a low CI value.